1. Field of the Invention
This invention relates generally to a nozzle, which directs a fluid stream to function as a jet pump for boosting automatic transmission oil pump inlet pressure using excess flow recirculation.
2. Description of the Prior Art
Many automatic transmissions include fixed-displacement oil pumps, which delivers a fixed volume of fluid for each cycle of the pump. However, the delivery of this volume is contingent upon the pumping chambers filling completely with oil during the intake portion of the pump's cycle. Typically, some speed is reached at which the pressure head in the inlet port is insufficient to force fluid into the pumping chambers at the rate at which the chamber volume is expanding. At this speed (known as the High Speed Fill Limit) the vapor pressure of the fluid is reached and the fluid begins to cavitate inside the pumping chamber. The level of cavitation increases with increasing speed.
This cavitation has several effects. It reduces the pump volumetric flow rate below its theoretical value. Generally, this decrease in flow rate relative to the theoretical value does not pose significant problems for the transmission from a flow availability standpoint, since fixed-displacement pumps are generally sized to provide sufficient flow at lower speeds and thus produce excess flow at high speeds.
Pump cavitation also causes noise. Continual increases in customer expectations for vehicle refinement mean that pump noise has become an area of significant interest in powertrain NVH, so cavitation noise can negatively impact customer satisfaction and result in a poor perception of vehicle quality.
High levels of cavitation can produce significant hardware damage if left unchecked, which is, of course, unacceptable.